(a) Field of the Invention
The present invention relates to a data transmitting and receiving apparatus and method for visible light communication, and more particularly, to the data transmitting and receiving apparatus and method for visible light communication, which can minimize the brightness reduction and mitigate the flicker of a light source in visible light communication.
(b) Description of the Related Art
Recently, as lighting apparatuses using light emitting diodes (LEDs) as light sources have spread, the visible light communication (VLC) technology for communication using LED lighting apparatuses is being introduced.
The VLC technology is a sort of wireless communication technology that transmits data through the visible light which human eye can detect. The VLC technology is different from the prior art fiber optic communication technology and infrared wireless communication technology in that it utilizes the visible light. Moreover, unlike radio frequency communications, the VLC technology is convenient because it can be freely used without any restriction or permission in terms of frequency use, and is different from other technologies as it offers excellent physical security and enables a user to see a communication link with their eyes.
In a VLC system using LED light sources, a transmitter generally uses the intensity modulation method or the on-off keying (00K) modulation method which varies the intensity of a visible light signal for modulating electrical data “0” and “1” because it is difficult for a receiver to detect the phase information of the transmitted signal and it is rather easy to configure these methods. Moreover, the transmitter of the VLC system using the LED light source may include a coding block for mapping digital data “0” and “1” into signal waveforms of “0” and “1” defined in the system. Depending on the system, non-return-to-zero (NRZ), return-to-zero (RZ), or Manchester code as a coding scheme, has been commonly used.
However, the OOK modulation method has a disadvantage that it drastically decreases the average optical output power of a light source. In other words, assuming that the number of data “0” and the number of data “1” are stochastically equal, the average output power of a visible light signal radiated from the transmitter in the NRZ-OOK or Manchester-OOK method is equal to the optical output power when DC power signal having an amplitude equal to half the amplitude of a data “1” signal is applied to a light source. Also, the average optical output power obtained through the RZ-OOK method is less than that obtained through the NRZ-OOK or Manchester-OOK methods. Of course, when OOK modulation is used, the average optical power of a light source can be increased by substantially increasing the amplitude of a modulation signal or by adding a DC component. However, these methods may cause an excess of allowable values for the LED light source driving and hence can shorten the lifetime of the light source.
Accordingly, when the OOK modulation is applied to the transmitter of the VLC system using the LED light source, it cannot provide the sufficient brightness that an LED light source can emit.
Moreover, VLC using lighting should be flicker-free for eye safety. As used herein, the term “flicker” means the fluctuation of the brightness of light that a human eye can detect. However, the NRZ-OOK and RZ-OOK methods, other than the Manchester-OOK method, may cause the flicker because the brightness of a visible light source representing “1” and “0” differs from each other, and the ratio of patterns “1” and “0” within an arbitrary data transmission slot varies with data combinations.
Among the conventional techniques, the Pulse Position Modulation (PPM) method, which has been actively used in the field of infrared wireless communication, always has the same average brightness of a light source for every symbols, similarly to the Manchester-OOK method. Thus, this method is known to have the flicker mitigation effect. However, the maximum average brightness of a light source that the PPM modulation can offer is no more than 50% at in the case of 2-PPM method which can represent one bit per symbol, and the higher the number of bits per symbol, the lower the maximum average brightness of the light source.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.